1. Field of the Invention
This invention relates to powder metallurgy, and in particular to a method of making a surface densified and hardened powder metal part.
2. Discussion of the Prior Art
Many components of machines are required to function in rolling and rubbing contact at high surface pressures. This action causes wear by a variety of well known mechanisms including: abrasion, scuffing (galling) and surface pitting (rolling contact fatigue). An economical and well-known method of manufacture of machine components is by powder metallurgy (P/M) which involves compaction of a blend of fine powders of appropriate composition in a set of tools that result in a precise shaped preform. The preform is then subjected to heat in a process called"sintering" which bonds the powder particles together and alloys the blend ingredients to form the desired microstructure. The sintered product can then be heat treated conventionally to harden the surface or whole part to increase wear resistance.
It has been found by experiment that wear resistance against heavy rolling contact requires a surface layer or skin of high integrity to withstand the subsurface micro scale cracks. These cracks eventually propagate and join together until a small fragment of surface material breaks away leaving a small pit. This process spreads to form larger areas of pitting. Eventually the machine operating noise becomes a problem or in extreme cases, the surface fails leading to mechanical breakdown of the machine. In order to raise the surface integrity of a P/M mechanical component such as a roller or gear or sprocket, the surface density must be increased to an appropriate level. This can be accomplished in several ways including raising the density by repressing the whole sintered product in the cold or heated condition. Alternatively the surface can be densified locally by a rolling action. In the case of a gear or sprocket, the latter involves rolling and meshing against a master gear or sprocket at higher pressure. This process requires an expensive precision master former which has limited life due to wear, and depth of densification is limited.